Electrochemical devices such as batteries and Electric Double Layer Capacitor (EDLC) commonly contain a sequence of functional components such as an anode, a separator, and a cathode. During charging, a negative potential is applied to the anode and a positive potential is applied to the cathode. When a device is discharged, electrons flow through an external circuit from the anode to the cathode. The anode and cathode are electrodes. A separator is used to prevent direct contact between the anode and cathode materials, such that electrons cannot conduct directly from anode to cathode within the device. Separators comprise materials with very low or negligible electronic conductivity. Electrodes and separators can be homogeneous functional materials, or composite materials comprising one or more functional materials in combination with additional materials such as binders or other fillers. In many electrochemical devices such as batteries or EDLC, the electrodes comprise particulates of the components. Other devices, such as thin film batteries or sintered solid state batteries comprise solid, monolithic electrode layers. Separators are often permeable mats or films, or similar structures, or solid state electrolyte (ion conductor) materials.
Common separators known in the art include porous films and nonwoven fiber mats. Separators in the art which contain particulates include particulate coatings coated onto a sheet of separator material (to prevent shorting if the separator melts or otherwise fails), and solid electrolytes with particulate fillers designed to improve mechanical properties, ionic conductivity, or chemical and thermal stability. In addition to conventional separators, porous particulate separators have been described for use in batteries. Batteries with liquid electrolytes are encased within a package (e.g. a can, coin cell package, pouch, or prismatic envelope). Liquid electrolyte resides within the package and permeates the separator. Liquid electrolyte is sometimes added to the package and separator, or simply to the package followed by permeation into the separator.
EDLC electrodes typically comprise substantial amounts of a high surface area carbon, such as activated carbon. During charging and discharging, ions from the electrolyte form a thin (e.g., monolayer) dielectric coating on the high surface area carbon electrode; no Faradaic reaction occurs.
Battery electrodes comprise materials which undergo Faradaic reactions during charging and discharging. For example, a lithium ion battery can comprise a lithium metal anode and a lithiated transition metal oxide cathode. During charging, lithium ions and electrons are extracted from the cathode (concurrent with oxidation of the transition metal ions), and combine at the anode to form lithium metal or other reduced lithium such as lithium in graphite.
In addition to basic symmetric EDLC devices, other capacitive devices can be constructed with an electrode that will undergo fast, reversible surface Faradaic reactions only. Such a device remains roughly capacitive-like in its electrical characteristic (charge proportional to voltage) and is therefore referred to as a pseudocapacitive device. Hence electrodes can be battery electrodes, basic EDLC electrodes, or pseudocapacitive electrodes.
Common electrochemical devices often comprise liquid electrolytes. These devices are often produced by fabricating electrodes separately, then assembling a structure around a prefabricated separator. The devices are sealed in can or pouch-like structures, and are often bulky and expensive. Solid state deposited or printed devices are also known. Generally, these devices utilize a solid state ion conductor. Such devices can be deposited by thin film methods, resulting in high cost and limited layer thickness. Alternatively, printed particulate devices can be made with solid state ion conductors. These devices can have limited rate capability and can be difficult to manufacture due to requirements for controlled humidity and other conditions.
There is a need for relatively low cost, easily manufactured devices that are customizable, flat, flexible, and have the high performance characteristics associated with liquid electrolytes.